Active pixel sensors (APS) are image sensors including integrated circuit containing an array of pixel sensors, each pixel containing a photodetector and an active amplifier. Such an image sensor is typically produced by a complementary metal-oxide-semiconductor (CMOS) process. CMOS APS can be used in web cams, high speed and motion capture cameras, digital radiography, endoscopy cameras, DSLRs, cell phone cameras, and the like. Other advances in image sensor technology have been implemented, such as the use of an intra-pixel charge transfer along with an in-pixel amplifier to achieve true correlated double sampling (CDS) and low temporal noise operation, and on-chip circuits for fixed-pattern noise reduction.
Some CMOS APS imagers have utilized backside illuminated (BSI) technology. BSI imager technology includes a semiconductor wafer bonded to a permanent carrier on the front side and then thinned from the backside. Passivation layers, anti-reflecting layers, color filters and micro-lens can be positioned on the backside, and the resulting device can be backside illuminated. Through-Silicon Vias (TSV) can be used to provide electrical connections from the front side to backside output pads. BSI CMOS APS imagers are becoming useful technology for many types of visible imagers in cell phones and digital cameras.
More generally, electromagnetic radiation can be present across a broad wavelength range, including visible range wavelengths (approximately 350 nm to 800 nm) and non-visible wavelengths (longer than about 800 nm or shorter than 350 nm). The infrared spectrum is often described as including a near infrared portion of the spectrum including wavelengths of approximately 800 to 1300 nm, a short wave infrared portion of the spectrum including wavelengths of approximately 1300 nm to 3 micrometers, and a mid to long range wave infrared (or thermal infrared) portion of the spectrum including wavelengths greater than about 3 micrometers up to about 20 micrometers. These are generally and collectively referred to herein as infrared portions of the electromagnetic spectrum unless otherwise noted.
Traditional silicon photodetecting imagers have limited light absorption/detection properties. For example, such silicon based detectors are mostly transparent to infrared light. While other mostly opaque materials (e.g. InGaAs) can be used to detect infrared electromagnetic radiation having wavelengths greater that about 1000 nm, silicon is still commonly used because it is relatively cheap to manufacture and can be used to detect wavelengths in the visible spectrum (i.e. visible light, 350 nm-800 nm). Traditional silicon materials require substantial path lengths and absorption depths to detect photons having wavelengths longer than approximately 700 nm. While visible light can be absorbed at relatively shallow depths in silicon, absorption of longer wavelengths (e.g. 900 nm) in silicon of a standard wafer depth (e.g. approximately 750 μm) is poor if at all.